Beryllium-aluminum-silver composite



May 30, 1967 TEM PERATURE, "C

R. H- KROCK ETAL BERYLLIUMALUMINUM-SILVER COMPOSITE Filed April 21, 1966 SILVER-ALUMINUM PHASE DIAGRAM WEIGHT PER CENT ALUMINIUM O 2 4 6 a Al ov-(AU 9|.l (72) O IO 2O 3O 4O 5O 60 7O 8O 90 I00 Ag ATOMIC PER CENT ALUMINUM A1 JFIGZ //Vl E/V7'0/?$ RICHARD H. KROCK EARL l. LARSEN CLINTFORD R. JONES ATTORNEY United States Patent G 3,322,512 BERYLLlUM-ALUMlNUM-SILVER COMPGSITE Richard H. Krock, Peabody, Mass, Earl I. Larsen, Indianapolis, Ind, and Clintford R. Jones, Arlington, Mass, assignors to P. R. Mallory & Co. Inc., Indianapolis,

Ind, a corporation of Delaware Filed Apr. 21, 1966, Ser. No. 544,182 3 Claims. (Cl. 29182.2)

ABSTRACT OF THE DISCLOSURE A ternary metal composite containing about 5085 percent, by Weight, beryllium, about 10.5 to 35 percent, by weight, aluminum and about 4.5 to percent, by weight, silver.

The present invention relates to composites of beryllium-aluminum-silver and more particularly to means and methods for providing such composites through liquid phase sintering.

Liquid phase sintering differs from the several other types of sintering techniques in that the sintering of the compact is carried out in the presence of a liquid phase. Liquid phase sintering encompasses raising the temperature of the compressed powder metal constituents to a temperature wherein a predetermined amount of the liquid phase appears. In the liquid phase, one of the metal constituents, the solid, is progressively dissolved in the other metal constituent, the liquid. However, the quantities of these constituents are such that, at equilibrium, some solid phase always exists. It is thought that the liqquid wets the solid so as to bring about favorable surface energies existing between the liquid and the solid thereby permitting solution into the liquid phase.

However, heretofore, when beryllium-alurninum-silver composites were developed in accordance with known liquid phase sintering techniques, it was found that the solid beryllium expelled the liquid aluminum-silver-beryllium alloy from the compact during liquid phase sintering. It is thought that the unfavorable surface energy equilibrium causing expulsion of the liquid is due to a tough, tenacious film of beryllium oxide which is present on each particle of beryllium.

The present invention prevents the expulsion of the liquid from the specimen by using an agency to intervene in the sintering stage. The agency either breaks down the oxide film on the beryllium or segregates to the metal oxide interface and lowers the surface energy of the liquid metal with respect to the beryllium oxide film so that the liquid metal progressively dissolves the solid metal.

The agency can be called a fluxing agent or flux, however, the agent has other characteristics which assist in wetting beryllium so as to surround the beryllium with a ductile envelope phase of an aluminum-silver-beryllium alloy matrix metal thereby avoiding the expulsion of the liquid from the specimen.

Beryllium has several desirable physical features which make it attractive for a variety of applications such as lightweight gears, lightweight fasteners, airplane parts or the like. However, beryllium has one major drawback which has seriously limited its commercial acceptance, that is, beryllium is inherently brittle at room temperature.

The lack of ductility of beryllium is attributed to the crystal structure of beryllium which is hexagonal close packed. During deformation, the basal planes of the hexagonal close packed structure, being the easiest to slip, are aligned along the working direction. Since slip is crystallographically difiicult perpendicular to the basal plane, the ductility of beryllium perpendicular to the primary fabrication direction is practically nonexistent.

Several tentative solutions have been advanced in an attempt to make beryllium metal sufiiciently ductile so as to permit a widespread commercial acceptance of the metal. Cross-rolling and cross-forging have been suggested as fabrication methods which might enhance the ductility of beryllium. These fabrication techniques reduced the number of basal planes along the direction of rolling and resulted in improved ductility. However, the degree of improvement was far from satisfactory. The fact remained that beryllium must be classified as brittle at room temperature even utilizing the aforementioned method when ductility perpendicular to the fabrication temperature is considered. In addition, the above mentioned technique would not be feasible where the fabrication is, by nature, solely along one axis such as swaging, drawing, and extrusion.

In recent years, attention has been directed to the fabrication of beryllium alloys not having the inherent brittleness of beryllium itself but possessing various outstanding properties of the metal such as, for example, low density combined with high strength. It is thought that US. Patent 3,082,521 fabricated the first ductile beryllium alloy by rapidly quenching the part from a temperature at which it was liquid. However, the beryllium content was not in excess of 86.3 atomic percent which is approximately weight percent. Although the beryllium alloy was ductile, the density of the alloy was in excess of that of aluminum and about equal to that of titanium.

It has also been suggested that beryllium alloys might be fabricated by pressing and sintering a mix of metal powders. However, such a method results in expulsion of the matrix metal or metals from the beryllium specimen and the eventual freezing of the matrix metal or metals into globs on the surface of the solid specimen. It is thought that the expulsion of the matrix metal or metals is due to the surface energies of the solid beryllium and the various liquids formed. The unfavorable surface energy equilibrium is believed to be due to a tough, tenacious film of beryllium oxide which is present on each particle of beryllium.

A means and method have been discovered for preparing a composite of beryllium, aluminum, and silver containing 50 to 85 percent, by weight, of beryllium, 10.5 to percent, by weight, aluminum and 4.5 to 15 percent, by weight, silver, thereby producing a composite having a density about the same as or less than that of aluminum, having high strength, and having good ductility. The ductility is due to the resulting microstructure of the composite. By surrounding the beryllium particles with a ductile envelope phase, a composite is formed where, under load, the beryllium is so constrained by the ductile phase that it and the ductile phase deform continuously.

The 85 percent, by weight, beryllium composite showed a considerable amount of particle contiguity and would represent, it is thought, an upper limit on the percent by weight of beryllium contained by the composite. Although no sagging was observed in the percent by weight of beryllium composite, a further decrease in beryllium content would raise, it is thought, the density value of the composite to a value of little interest. The aluminum and the silver are in a ratio of a least 50 parts aluminum by weight in this ratio as the beryllium content of the composite is modified within the hereinbefore disclosed limits. It is thought that the ratio of aluminum to silver may be varied without having any substantial adverse effect on the properties of the composite.

Alkali and alkaline earth halogenide agents such as lithium fluoride-lithium chloride or the like in a determined ratio are utilized to segregate to the solid interface of the beryllium particle and either break down the film on the particle of beryllium and/or alter the liquid-solid surface energy in the system.

Therefore,- it is an object of, the presentinvention to provide anagentto promote. liquid phase sintering of a bfi 'lllliumaluminumsilver mixture A further object of the present invention is to provide a ductile beryllium composite having alow. density andhigh strength.

Another object of thBPIESGDflIIVCHllQHIS to'provide a ductile beryllium composite having a matrix phase that is heat treatable.

A further object of'the present invention is to provule a ductile compositeof berylliumin which beryllium is the predominate ingredient. Another object of the. present invention is to provide a 1 means and method of producing a ductile composite of I beryllium-aluminumsilver whose .microstructure consistsof berylliumparticles surrounded by a ductileenvelope A phase of an aluminurnsilver-.beryllium alloy matrix metal.

I Yet another object of the present invention is to provide a ductile composite of. beryllium-aluminum silver contain from a berylliumspecimen.

Still 'another'object of the present inventionisto provide alkali and alkaline earthhalogenide agents used in the fabrication of a beryllium composite.

Another-object of the present invention is to provide a composite of berylliurn alurninum-silver that. may be sin tered to substantially theoretical density.

' Yet another object of the present inventioni'sto provide I a'rneans and method whereby a'du'ctile' beryllium composite may be successfully fabricated in both a practical and economical manner.

' Afurther object of the present invention 'isto providea lithium fluoride-lithium chloride agent for promoting liquid phase sintering in aberyll-ium, aluminum, andsilver mix.

Yet still another object of the present invention is to provide a lithium fluoride-lithium chloride agent wherein the constituents are used in a predetermined ratio.

The present invention, in another of its aspects, relates to novel features of the instrumentalities of the invention described herein for teaching the principal object of the invention and to the novel principles employed in the instrumentalities whether or not these features and principles may be used in the said object and/or in the said field.

With the aforementioned objects enumerated, other objects will be apparent to those persons possessing rdinary skill in the art. Other objects will appear in the following description and appended claims. The invention resides in the novel combination of elements and in the means and method of achieving the combination as hereinafter described and more particularly as defined in the appended claims.

In the drawings:

FIGURE 1 is a phase diagram for binary alloys of aluminum-silver.

FIGURE 2 is a photomicrograph of a beryllium specimen illustrating a matrix metal expelled from the specimen by the forces of surface energy of solid beryllium and various liquids formed.

FIGURE 3 is a photomicrograph of a 70 percent, by weight, beryllium, 21 percent, by weight, aluminum, remainder silver composite illustrating beryllium particles surrounded by a ductile envelope phase of an aluminumsilver-beryllium alloy.

Generally speaking, the means and method of the present invention relate to a ductile beryllium composite fabricated by liquid phase sintering. The composite contains about 50-85 percent, by weight, of beryllium. 10.5-

' silver composite by liquid phase sintering comprises. the. g .steps of mixing predetermined portions of powder .beryllium and powder alloy of aluminum, silver, or aluminum powder and silver powder with a predetermined portion of an agent selected from the group consisting of alkali that the alloy progressively dissolves the berylliumat the sinteringtemperature. Thereafter, the composite may be heat treatedand rapidly quenched so that the heat-treating temperature structure is preserved; and the aluminum is supersaturated with silver. g j g n .More particularly, the method ofthepresent invention comprises mixing powder beryllium of -about50 '85 per- I cent, by weighgwith a powder alloy of aluminumand:

silverin a 7to 3 ratio.-An agent of lithium fluoride-lithium 35 percent, by weight, aluminum, and 4.5 to percent,

by weight, silver.

The method of producing the beryllium aluminumand alkaline earth .halogenides. The portions are pressed in a die to form a green compact. The compact is then heated to the sintering temperature. At this temperature the agent provides a favorable. surface energy equilibrium between the beryllium and the aluminum-silver alloy. so

i chloride in about 0.5Ito 2.0 percent, by weight, of'the total metal additions'is. mixed with the beryllium andthealloy powder or elemental powder. The constituents of the.

.agentare in about a: one to'onerlatio by weight. The beryb lium, the alloy powder or elemental powdenand the agent are pressed so as to form a green compact. The green compact is heatedin a non-oxidizing atmosphere suchas.

argon at a temperature of about. 1000" centigrade to about 1100 centigrade. At the aforementioned temperar 'tures, the agent provides a favorable surfaceenergy equilibrium between the beryllium and the alloy so that the. aluminum-silver alloy progressively dissolves the heryl-- I iiurn;The microstructureofthe'resultant compositeconsists of berylliumparticles surrounded by a ductile euvelope phase of an aluminum-silver-beryllium alloymatrix metal. The alloy issintered. to substantially itstheoretical density; The alloy may be specially heat-treated and rapidly quenched .so that the heattreating temperature struc-. ture is preserved and the aluminum is supersaturated with silver.

In carrying out the present invention, a beryllium base compact is fabricated by any suitable means such as powder metallurgy techniques. A suggested method utilizing this technique is to mix beryllium powder with an alloy of aluminum-silver or the elemental powders and an agent of equal parts of lithium fluoride-lithium chloride. The powders are blended and mixed by ball milling the metal powders and the flux agent. The blended and mixed powders are compacted to form a green compact by accepted metallurgical methods such as by compacting within the confines of a dies or a hydraulic or an automatic press or by placing the powders in a rubber or a plastic mold and compacting in a hydrostatic press. The green compact is sintered in a non-oxidizing atmosphere such as argon or the like at a temperature of about 1000 centigrade to about 1100 centrigrade. It is seen that the range of the sintering temperatures is below the 1277 centigrade melting point temperature of beryllium but above the 620 centigrade melting point temperature of the aluminum-silver alloy. The aluminum-silver alloy will dissolve smaller beryllium particles and will dissolve the surfaces of the larger beryllium powder particles thereby surrounding the remaining beryllium particles with a ductile envelope phase of aluminum-silver-beryllium alloy during sintering of the compact.

The agent, lithium fluoride-lithium chloride, either breaks down the oxide film on the beryllium or segregates to the metal oxide interface lowering the surface energy of the liquid metal with respect to the beryllium oxide film. Simply, the agent causes the liquid to wet the beryllium.

Composites containing about 50 to percent, by weight, of beryllium, and the remainder an alloy of aluminum-silver were successfully fabricated. The agent prevented the expulsion of the liquid aluminum-silverberyllium alloy from the compact by the forces of surface energy, that is, prevented the formation of very fine rounded droplets of the aluminum-silver-beryllium alloy on the surface of the beryllium specimen. FIGURE 2 shows a beryllium specimen 20 having on the surface thereof an expelled alloy 21 of aluminum-silver-beryllium. Specimens from which the aluminum-silver-beryllium alloy has been expelled have gross porosity and as a result are weak, brittle, and of little commercial value.

The composition of the agent utilized is about 50 parts, by weight, of lithium fluoride to about 50 parts, by weight, of lithium chloride. The agent provides an action, such that, upon heating or sintering of the pressed powder mix to the temperature at which the liquid phase forms, expulsion of the melt from the specimen is eliminated. Furthermore, it was found that solution of the beryllium into the alloy was enhanced as evidenced by the rounded particles of beryllium in the microstructure.

It was found that the amount by weight of lithium fluoride-lithium chloride agent should exceed 0.5 percent, by weight, of the total of all metal additions. It would appear that the optimum range of the agent is from about 0.5 percent to about 2.0 percent, by weight, of the total of all metal additions. It is believed that the quantity of lithium fluoride-lithium chloride agent required is related to the amount necessary to cover the total beryllium surface area. Hence, the minimum amount of agent needed 'would be a function of the surface area of the beryllium powder. The utilization of lithium fluoride-lithium chloride agent in other than equal parts is possible. It is thought, however, that an equal parts mixture achieves optimum results.

By using the methods of the present invention and the lithium fluoride-lithium chloride agent, compacts were fabricated containing up to 85 percent, by weight, of beryllium, the remainder an alloy of aluminum-silver without the use of pressure during sintering. The compo-s ite was sintered to between about 88 and 92 percent of its theoretical density by a single sinter and achieved about 96 percent of theoretical density by a double repress and an intermediate re-liquid phase sinter. The good strength and low density characteristics of the beryllium were retained and the resulting beryllium-aluminum-silver composite possessed good ductility.

Thus, by substantially surrounding the beryllium particles with a ductile envelope phase of an aluminum-silverberyllium alloy matrix metal, the beryllium and the matrix metal deform continuously under load.

An aluminum-silver phase diagram is illustrated in FIGURE 1.

Silver is an effective material for hardening aluminum. The theory of the deformation of dispersed particle composite materials states that ductility in such a composite will be enhanced when the constrained flow stress of the matrix phase can be made as equal as possible to the flow stress of the dispersed particles. Hence, silver is used to harden aluminum. Once the composite has been cooled to room temperature, the effectiveness of the silver is brought into play by a subsequent heat treatment. The phases present in the beryllium-aluminum-silver system at room temperature consist of beryllium particles dispersed in an aluminum-silver matrix in which some residual zeta phase containing 40' atomic percent aluminum may be dispersed. In order to most elfectively harden the material, the composite is heated into the complete alpha aluminum phase. It was found that heat treating the composite between 500 and 570 centigrade for about 1 to 2 hours is sufficient to completely dissolve all the silver in the aluminum. The composite is rapidly quenched into a satisfactory medium such as water or the like, such that the high temperature structure is preserved and the aluminum is supersaturated with silver. Hence, the solutionizing treatment contains all the silver in solution, whereas at equilibrium, zero percent silver in aluminum is called for. The silver can be precipitated out of the supersaturated solid solution as a zeta phase increasing the strength of the aluminumsilver matrix. A distinct advantage of the berylliumaluminum-silver composite is that the matrix phase is heat treatable.

Attention is directed to FIGURE 3, wherein a photomicrograph of 500 magnifications shows a composite of 30 percent, by weight, aluminum-silver alloy in beryllium after being etched by any suitable etching means such as a dilute solution of ammonium hydroxide and hydrogen peroxide. The areas 10 are beryllium particles. The areas 11 are the aluminum-silver-beryllium alloy surrounding the beryllium particles.

Example 1 shows the expulsion of the liquid from a beryllium specimen and Examples 2-8 are illustrative of the preparation of beryllium-aluminum-silver composites by liquid phase sintering.

EXAMPLE 1 Expulsion of the liquid aluminium-silver-beryllium alloy from the solid beryllium specimen during liquid phase sintering when the agent of lithium fluoride-lithium chloride is not used in the preparation of a berylliumaluminum-silver composite.

A mixture of about 70 percent, by weight of beryllium having a particle size of 200 mesh or finer was ball mill mixed with about 30 percent, by weight, of an alloy of aluminum-silver or the elemental powder of suitable particle size. The alloy contains 70 percent, by weight, aluminum and 30 percent, by weight, silver. The milled mixture was pressed by any suitable means such as by an automatic press at a suitable pressure to provide a green compact sturdy enough to be handled. It was found that pressures of from about 15,000 to 20,000 pounds per square inch resulted in a green compact having a density from about 50 to 60 percent of theoretical density and sufiiciently strong to be handled. Sintering of the compact was carried out in an argon atmosphere at about 1100 centrigrade for about 1 hour. This technique, due to the surface energies of the solid beryllium and the liquid formed, resulted in the expulsion of the liquid from the specimen and its eventual freezing into rounded globs on the surface of the specimen as shown in FIGURE 2.

EXAMPLE 2 A composite of about 70 percent, by weight, beryllium, 21 percent, by weight, aluminum, and the remainder silver.

A mixture of about 70 percent, by weight, of beryllium powder having a particle size of 200 mesh or finer was ball mill mixed with about 30 percent, by weight, of an alloy of aluminum-silver powder of suitable particle size. The alloy contains 70 percent, by weight, aluminum and 30 percent, by weight, silver. Also ball mill mixed with the beryllium and alloy powders was about 1.0 percent, by weight, of the total metal additions equal parts of an agent of lithium fluoride-lithium chloride. Mixtures of the beryllium and alloy powders were also prepared with the agent having 05 and 2.0 percent, by weight, of the total metal additions. The milled mixture was pressed by any suitable means such as by an automatic press at a suitable pressure to provide a green compact sturdy enough to be handled. It was found that pressures of from about 15,000 to 20,000 pounds per square inch resulted in a green compact having a density of from about 50 to 60 percent of theoretical density and sufiiciently strong to be handled. Sintering of the compact was carried out in an argon atmosphere at about 1000 centigrade for about 1 hour. The composite is heattreated at about 570 centigrade for about 1 hour so as to completely dissolve all the silver into the aluminum. The composite is then rapidly quenched so that the heattreating temperature structure is preserved and the aluminum is supersaturated with silver. The solutionizing treatment contains all of the silver in solution. The silver can be precipitated from the supersaturated solid solution as a zeta phase (see FIGURE 1). The microstructure of FIGURE 3 was produced using the abovementioned procedure.

EXAMPLE 3 A composite of about 70 percent, by weight, beryllium, 21 percent, by weight, aluminum, and the remainder silver.

The procedure of Example 2 was followed using 70 percent, by weight, beryllium powder, 21 percent, by weight, aluminum powder, and the remainder silver powder. Individual composites were prepared using 0.5, 1.0 and 2.0 percent, by weight, of the total metal additions.

EXAMPLE 4 A composite of about 70 percent, by weight, beryllium, 21 percent, by weight, aluminum, and the remainder silver.

The procedure of Example 2 was followed using 70 percent, by weight, beryllium powder, mixed with about 30 percent, by weight, of an alloy powder of aluminumsilver. The alloy contains 70 percent, by weight, aluminum and 30 percent, 'by weight, silver. Individual cornposites were prepared using 0.5, 1.0 and 2.0 percent by weight of the total metal additions of the agent lithium fluoride-lithium chloride at a temperaure of about 1100 centrigrade using the aforementioned procedure.

EXAMPLE 5 A composite of about 50 percent, by weight, beryllium, 35 percent, by weight, aluminum, and the remainder silyer.

The procedure of Example 2 was followed using 50 percent, by weight, beryllium powder, mixed with about 50 percent, by weight, of an alloy powder of aluminumsilver. The alloy contains 70 percent, by weight, aluminum and 30 percent, by Weight, silver. Individual composites were prepared using 0.5, 1.0 and 2.0 percent by weight of the total metal additions of the agent lithium fluoride-lithium chloride at temperatures of about 1000 and 1100" centigrade using the aforementioned procedure.

EXAMPLE 6 8 EXAMPLE 7 A composite of about percent, by weight, beryllium, 17.5 percent, by weight, aluminum, and the remainder silver.

The procedure of Example 2 was followed using 75 percent, by weight, beryllium powder, mixed with about 25 percent, by weight, of an alloy powder of aluminumsilver. The alloy contains 70 percent, by weight, aluminum and 30 percent, by weight, silver. Individual composites were prepared using 0.5, 1.0 and 2.0 percent by weight of the total metal additions of the agent lithium fluoride-lithium chloride at temperatures of about 1000 and ll00 centigrade using the aforementioned procedure.

EXAMPLE 8 A composite of about percent, by weight, beryllium, 15 percent, by weight, aluminum, and the remainder silver.

The procedure of Example 2 was followed using 85 percent, by weight, beryllium powder, mixed with about 15 percent, by weight, of an alloy powder of aluminumsilver. The alloy contains 70 percent, by weight, aluminum and 30 percent, by weight, silver, Individual composites were prepared using 0.5, 1.0 and 2.0 percent by weight, of the total metal additions of the agent lithium fluoride-lithium chloride at temperatures of about 1000 and 1lO0 centigrade using the aforementioned procedure.

The present invention is not intended to be limited to the disclosure herein, and the changes and modifications may be made in the dis-closure by those skilled in the art without departing from the spirit and scope of the novel concepts of this invention. Such modifications and variations are considered to be within the purview and scope of this invention and the appended claims.

Having thus described our invention, we claim:

1. A ternary metal composite containing about 70 percent, by weight, beryllium and the remainder an alloy of aluminum-silver wherein said alloy contains about 70 percent, by weight, aluminum and the remainder silver.

2. A ternary metal composite containing about 50-85 percent, by weight, beryllium and the remainder an alloy of aluminum-silver, said alloy containing about 70 percent, by weight, aluminum and the remainder silver.

3. A ternary metal composite containing about 5085 percent, by weight, beryllium, about 10.5 to 35 percent, by weight, aluminum and about 4.5 to 15 percent, by weight, silver.

References Cited CARL D. QUARFORTH, Primary Examiner.

L. DEWAYNE RUTLEDGE, Examiner.

R. L. GRUDZIECKI, Assistant Examiner. 

2. A TERNARY METAL COMPOSITE CONTAINING ABOUT 50-85 PERCENT, BY WEIGHT, BERYLLIUM AND THE REMAINDER AN ALLOY OF ALUMINUM-SILVER, SAID ALLOY CONTAINING ABOUT 70 PERCENT, BY WEIGHT, ALUMINUM AND THE REMAINDER SILVER. 